1. Field of the Invention
This invention concerns methods of radiant energy heating of surfaces and apparatus therefor and also their application to such uses as thermal sealing techniques which utilize the direct impingment of radiant energy on the surfaces in order to heat a thermoplastic or other thermally activated adhesive prior to joining or lapping two surfaces which are to be bonded or sealed.
2. Description of the Prior Art
Heat sealing of the surfaces of lapped sheets such as the flaps of cartons has long been known and practiced in the art. As practiced, such heat sealing may take various forms as in the application of a thermoplastic adhesive to one or the other or both of the lapped sheets and the application of heat causing the material to become plastic and with the sheets then lapped as by folding the flaps of the container and upon cooling a sealing of the container flap is obtained. Alternatively, the sheet material may be coated with wax or other thermoplastic coating such as polyethylene which upon being heated acts as a sealant.
The term "thermoplastic" is here being used in the broadest sense, i.e., to indicate any material which softens upon being heated and hardens upon cooling. This is commonly done by the application of heat by means of heating "shoes" which engage the sheet material. However, any contact of the shoe with the thermoplastic coating or adhesive coated side of the sheet causes the shoe to scrape the thermoplastic material from the sheet. Thus, with this approach, it is usual to heat the opposite side of the sheet with the shoe in contact therewith which is inefficient use of the heat energy and also creates a tendency to char the paper since it is difficult to adjust the heat level correctly. The mechanical nature of the system also leads to troublesome maintenance problems in factory installations of the apparatus.
This further precludes the use of board stock which is polyethylene coated on both sides. Double coated polyethylene is required for air exclusion (barrier properties) and for creating packages which may be sealed without the added step of applying (or pre-applying and activating) an adhesive.
Open flame heating is also in current use which offers the advantage of rapid shutoff, but has obvious safety drawbacks.
It has thus heretofore been proposed and practiced that the heating of a thermoplastic material bearing areas of the sheet be carried out by means of the direct application of radiant energy to the surface of the sheet such as is disclosed in U.S. Pat. No. 3,614,914 to Troll. In the arrangement disclosed in that patent, a source of radiant energy is disposed so as to heat thermoplastic material coating areas of the container flap as it passes along a conveyor line prior to the flap being folded to produce a sealed carton. The radiant emissions of the source of radiant energy is, as has usually been utilized to carry out the heating of the material, in the infrared wavelength range. In addition, the heat sources are disclosed as being of a type such that they are "area" sources as distinguished from a "point" source and further involve radiant energy sources which have large mass heating elements which have a high thermal inertia in that the temperature of the elements cannot be quickly changed to vary the intensity of the radiation emission level.
The approach described in the Troll patent can be said to be typical of conventional practice in these three respects, i.e.: the use of infrared range wavelengths of radiant energy; the use of an "area" type radiant energy source; and, the use of high thermal inertia sources.
These characteristics lead to several disadvantages in this context. Firstly, the absorbency of infrared radiation is difficult to control in that such relatively long wavelength radiations tend to penetrate most materials and their energy is absorbed only after passing through a substantial thickness of material. Thus, if one wishes to concentrate the heat energy in the adhesive layer or at the interface between the adhesive layer and the backing sheet, this cannot be very successfully done with infrared radiation. Furthermore, if one attempted to control the area of absorption by placing markings on the sheet material to be sealed, the penetrating characteristics of infrared radiation is such that excessively thick coatings would be required on the sheets.
Alternatively, backing sheets would be required such as are disclosed in U.S. Pat. No. 3,247,041 which discloses an infrared radiant energy source which is absorbed selectively in discrete areas of a backing sheet to indirectly heat the overlying sheets to be sealed together in the region of the marking pattern. Another difficulty encountered in the use of infrared radiation and also in the use of area sources or radiant energy is the impossibility or at least great practical problem of focusing the radiant energy. Although crude reflectors are used to concentrate the radiation, no true focusing of the radiation is possible due to the relatively large area of the source. Such focusing capability would potentially have great utility in this area since firstly more efficient use of the radiant energy would be possible allowing the use of lower-powered sources which would tend to allow lower thermal inertia sources since smaller filament masses which operate at higher temperatures could be utilized; secondly, the area upon which the radiant energy is directed could be quickly varied for applications calling for different areas to be heated by the source by merely adjusting the optics and rapid pattern change could be achieved. Also such variations would allow a ready change in the intensity of the radiation impinging on the surface.
In many cases, the mechanical components of such systems could be simplified by the expedient of focusing the radiant energy.
While infrared radiation can theoretically be focused, it requires elaborate and exotic optical components such as gold-plated reflectors or sodium chloride crystal lens, since the absorption of long wavelength energy by conventional glass lens elements tends to render them inefficient or impractical, particularly at energy levels which would be required for these applications.
These inefficiencies in the application of heat energy by radiant energy sources has heretofore led to the use of high-powered and simultaneously high thermal inertia sources such as quartz rod heaters or heating elements available under the trademark Cal Rod which leads to major difficulties in control over the heat energy being absorbed or applied to the surface heated. Commonly, the stoppage of a continuously operating sealing apparatus, in which the coated sheets or packages are being moved by means of a conveyor or other transport means past the radiation source, leads to overheating and even combustion of the sheets upon stoppage of the sheet material since the high thermal inertia radiant energy source cannot be de-energized with sufficient rapidity to prevent this result.
Another disadvantage is that the relative inefficiency of heating required the heating element to be placed in very close proximity to the surface heated, precluding the use of shutters or other control devices and increasing the fire hazards.
In addition, it would be advantageous if the emissions intensity of the energy source could be varied with sufficient rapidity to continuously correlate the velocity of movement of the sheet material past the energy source with the degree of energy emission of the radiant energy source.
The difficulty in controlling the application pattern and absorption of infrared radiant energy also contributes to the impossibility of controlling the specific areas within which the heating (and sealing) process is carried out, to thus preclude convenient separation of the sealed sheets in those instances where it is desirable that the seal be readily broken, i.e., to open the carton. The thermoplastic material tends typically to be heated indiscriminately by the massive levels of impinging radiant energy to a point where a complete fusing of the layers takes place and it is unfeasible to leave an unsealed area in those instances where the lapped sheets are completely coated with a thermoplastic material such as is common in the plastic coated milk cartons, ice cream cartons or bakery boxes in common use.
Other approaches have been proposed such as described in U.S. Pat. No. 3,461,014 in which the selective absorption is accomplished in the context of a microwave radiant energy source and in which a pattern is placed between the sheets to be sealed which pattern is formed with a microwave radiation absorbing material such as iron oxide dispersed in an ink or dye. However, the use of microwave equipment is not without its difficulties requiring shielding, special safety precautions and is relatively expensive. The use of special additives to the ink would also contribute to increased expenses in the container manufacture albeit this approach does contribute to the very accurate control over the areas on the sheets which receive and absorb the radiant energy and in addition is absorbed at the precise point at which maximum heat energy is desired.
Accordingly, it is an object of the present invention to provide a method and apparatus for efficiently heating discrete areas of a surface as for thermal sealing of lapped sheets by the direct application of radiant energy to at least one and generally both of the lapped sheets in order to soften a thermoplastic material carried on the surface into a plastic condition prior to sealing together the lapped sheets.
It is another object of the present invention to provide such a method and apparatus in which the absorption of the radiant energy may be relatively precisely controlled both as to the area of the surface heated and also as to the depth of the surface at which maximum absorption occurs.
It is yet another object of the present invention to provide such a method and apparatus in which the application of the radiant energy to the thermoplastic material can be very rapidly varied in order to discontinue the heating of the thermoplastic material or so that the degree of heating may be rapidly varied in accordance with the requirements of the particular application.
It is yet another object of the present invention to provide such a method and apparatus in which the area upon which the radiant energy is directed may be readily varied.
It is still another object of the present invention to provide a method and apparatus in which the radiant energy source is inexpensive and safe so as to minimize the safety hazards in the use of the source and also the capital expense involved in providing production facilities.
It is another object of the present invention to provide a means of improving efficiency of the radiant energy heating by enhancing the absorption of radiant energy within discrete areas by means of ordinary printing inks or dyes commonly in use in the printing and production of paperboard materials commonly used in boxes and other paperboard containers of the sort to which the present method and apparatus have particular application.